1. Field of the Invention
The present invention relates to a zoom lens and an optical apparatus having the same and, more particularly, to a zoom lens of a high magnification range suited to be used in single-lens reflex cameras or video cameras.
2. Description of Related Art
For zoom lenses, there have been known a focusing method of moving the first lens unit, or the so-called "front focus" method, and another focusing method of moving the second or later lens unit, or the so-called "inner focus" or "rear focus" method.
In general, the zoom lenses of the inner or rear focus type have their first lens units made smaller in diameter to admit of the effective light beam, than when the front focus type is in use. So, there is an advantage of assuring improvements of the compact form of the entire lens system. Moreover, the focusing lens unit is relatively small in size and light in weight. So, particularly for the auto-focus cameras that have recently become the main stream of development, fast focusing becomes possible to perform. Even this constitutes a characteristic feature of the inner or rear focus type zoom lens.
As one of the zoom lenses of such inner or rear focus type, mention may be made of the zoom lens that comprises, in order from an object side, a positive first lens unit, a negative second lens unit and succeeding lens units whose overall power is positive. All the separations between the adjacent lens units are made to vary to effect zooming. In this so-called "positive lead" type of zoom lens, the second lens unit of negative refractive power is used for focusing purposes, as disclosed in, for example, Japanese Laid-Open Patent Applications No. Hei 3-228008 (corresponding to U.S. Pat. No. 5,144,488), No. Hei 5-119260 (corresponding to U.S. Pat. No. 5,528,427), and No. Hei 6-230285.
This method, particularly when applied to the high range zoom lens including the standard region, not only brings the above-described features into full play, but also maintains good stability of optical performance throughout the entire focusing range from an infinitely distant object to a closest object.
Here, for the positive lead type of zoom lens, an explanation is made about the variation of magnification of the negative second lens unit during zooming of the entire zoom lens.
In general, the negative second lens unit has a negative fractional magnification at the wide-angle end and, as zooming advances to the telephoto end, the absolute value of the magnification increases. Moreover, since the second lens unit is the main variator of the positive lead type zoom lens, the magnification of the second lens unit increases by a large amount (or changes in such a direction as to approach "-1" from the negative fractional magnification) during zooming of the entire zoom lens from the wide-angle end to the telephoto end. Particularly in the case of the high range zoom lens, such an increase becomes conspicuous.
Now, the relationship between the magnification of the focusing lens unit and the focusing sensitivity (the ratio of the amount of shift of the focal plane to the amount of movement of the focusing lens unit) can be expressed by the following formula: EQU ES=(1-.beta.f.sup.2).times..beta.r.sup.2
where
ES: the focusing sensitivity, PA1 .beta.f: the magnification of the focusing lens unit, and PA1 .beta.r: the combined magnification of all the lens units disposed on the image side of the focusing lens unit.
It is understandable from the formula described above that, when the absolute value of magnification of the focusing lens unit is "1", the focusing sensitivity takes a value of "0", and as the absolute value of magnification of the focusing lens unit departs from "1", the focusing sensitivity becomes greater.
In the positive lead type of zoom lens, however, as described before, the magnification of the negative second lens unit varies in a direction from the negative fractional magnification to "-1" during zooming of the entire zoom lens from the wide-angle end to the telephoto end. For this reason, when shooting a close object with a zoom lens of high range, the focusing sensitivity of the negative second lens unit becomes small at or near the telephoto end, so that the total focusing movement is remarkably increased. Further, in a case where the magnification of the negative second lens unit takes a value of "-1" in a middle way of zooming, the focusing sensitivity becomes "0", so that it happens that the focusing becomes impossible.